Water is the chief fire fighting medium on a ship and the fire main is the basic installation for fighting fires. The system shown below has two independently powered pumps which are also used for general service and ballast. These pumps supply two engine room hydrants and the deck main through the isolating valve.
The later is required in the fire main system to prevent loss of water through damaged pipe work in the engine room if, to maintain the deck supply, the emergency fire pumps has to be used. The emergency fire pump is shown as being situated in the tunnel, with a supply to deck fire main through tunnel escape and also to both hydrants in the shaft tunnel by engine room water tight door. The deck main has a drain at the lowest position so that the pipe can be emptied (particularly of freshwater) in cold weather. If this is not done, the pipe can be damaged by water freezing, but more important he pipe will be blocked by the ice and not stable.
“General Engineering Knowledge” by H.D. McGeorge